Is it permissible for a husband to divorce one of his two wives (the first or the second) in order to preserve one household, given that his income is insufficient to support two families, and his realization that his second marriage was a mistake?

Your husband is absolutely permitted to divorce his second wife, but it is not permissible for you to request her divorce. This is based on the Prophet, peace and blessings be upon him, who said: "It is not lawful for a woman to ask for the divorce of her sister (in faith) in order to empty her plate, for she will only have what has been decreed for her." He is permitted to keep both wives, provided he is just. Either wife may waive her rights. If your waiting period has ended and you fear problems upon returning, then it might be more beneficial for you not to return.